Elevated CO₂ does not stimulate carbon sink in a semi-arid grassland

Abstract

Elevated CO2 is widely accepted to enhance terrestrial carbon sink, especially in arid and semi‐arid regions. However, great uncertainties exist for the CO2 fertilisation effects, particularly when its interactions with other global change factors are considered. A four‐factor (CO2, temperature, precipitation and nitrogen) experiment revealed that elevated CO2 did not affect either gross ecosystem productivity or ecosystem respiration, and consequently resulted in no changes of net ecosystem productivity in a semi‐arid grassland despite whether temperature, precipitation and nitrogen were elevated or not. The observations could be primarily attributable to the offset of ecosystem carbon uptake by enhanced soil carbon release under CO2 enrichment. Our findings indicate that arid and semi‐arid ecosystems may not be sensitive to CO2 enrichment as previously expected and highlight the urgent need to incorporate this mechanism into most IPCC carbon‐cycle models for convincing projection of terrestrial carbon sink and its feedback to climate change.

A meta-analysis of 1,119 manipulative experiments on terrestrial carbon-cycling responses to global change

“…2h; Q B = 5.70, P = 0.02). Similar to the observed increases in gross ecosystem productivity, ecosystem and soil respiration were also enhanced under eCO 2 , indicating that eCO 2 accelerates ecosystem carbon turnover through belowground processes (for example, microbial decomposition and soil CO 2 efflux) [49][50][51] . Root exudation and possibly root litter and organic matter decomposition might be stimulated under eCO 2 , thus a considerable fraction of increased net primary productivity is in fast-turnover pools that do not lead to carbon sequestration but are released back to the atmosphere.…”

Nitrogen deposition magnifies the sensitivity of desert steppe plant communities to large changes in precipitation

“…Similarly, a recent study reported that N deposition can increase the sensitivity of plants to climatic changes such as attendant drought (Hess et al, ). Moreover, a water deficit can result in the allocation of more plant N to sink organs such as root systems (Xu, Zhou, & Wang, ) and result in a decrease in soil available N through plants absorption (Yuan et al, ); in contrast, enhanced precipitation might easily result in N consumption in drylands (Ren et al, ; Song et al, ). In a grassland ecosystem in New Zealand, an accelerated N‐cycling process was observed with increased precipitation, promoting plant community functioning (de Klein, Shepherd, & van der Weerden, ).…”

“…Moreover, a water deficit can result in the allocation of more plant N to sink organs such as root systems (Xu, Zhou, & Wang, 2007) and result in a decrease in soil available N through plants absorption (Yuan et al, 2006); in contrast, enhanced precipitation might easily result in N consumption in drylands (Ren et al, 2017;Song et al, 2019).…”

“…Furthermore, the response of plant communities to N addition could be mediated by environmental factors, particularly precipitation (Reichmann, Sala, Peters, & Debra, ; Tian et al, ). Soil N availability varies greatly across time, especially in semi‐arid and arid ecosystems, largely owing to temporal fluctuations in precipitation (Austin et al, ; Song et al, ). Precipitation could influence N mineralization rates and plant N uptake (Gebauer & Ehleringer, ), thereby regulating N turnover and its effects on plant communities (Dijkstra et al, ; McCulley, Burke, & Lauenroth, ).…”

“…Soil N availability varies greatly across time, especially in semi-arid and arid ecosystems, largely owing to temporal fluctuations in precipitation (Austin et al, 2004;Song et al, 2019). Precipitation could influence N mineralization rates and plant N uptake (Gebauer & Ehleringer, 2000), thereby regulating N turnover and its effects on plant communities (Dijkstra et al, 2018;McCulley, Burke, & Lauenroth, 2009).…”